(1) Field of the Invention
The present invention relates to Lactococcus which produce at least two bacteriocins and to compositions incorporating the bacteriocins. In particular, the present invention relates to Lactococcus which contain DNA transferred from a donor Lactococcus which provides a bacteriocin along with a bacteriocin from a recipient Lactococcus strain.
(2) Prior Art
The genus Lactococcus includes dairy lactic streptococci that belong to the Lancefield serological group N. The species involved are Lactococcus lactis subsp. lactis, Lactococcus lactis subsp. cremoris, and citrate-fermenting Lactococcus lactis subsp. lactis biovar diacetylactis (Schleifer, K. H., FEMS Microbiol. Rev. 46(3):201-203 (1987)). The aforementioned species are traditionally used in dairy fermentations and are "generally regarded as safe" (GRAS) by the United States Department of Agriculture.
Certain strains of group N Lactococcus spp. produce proteinaceous antagonistic substances against closely related bacteria called bacteriocins (Klaenhammer, T. R., Biochimie 60(3):337-349 (1988)). Most of these bacteriocins have a narrow spectrum of activity either antagonistic to other strains within the species or to certain strains in closely related species. Nisin, a bacteriocin produced by certain strains of Lactococcus lactis subsp. lactis, however, has a wider spectrum of activity affecting other Gram-positive bacteria including the spore-forming Clostridia and Bacillus spp. (Delves-Broughton., Food Technol. 44(11):100-112, 117 (1990)).
Because of the traditional use of the dairy lactococci in various food fermentations (Gilliland, S. E., Bacterial Starter Cultures for Foods. CRC Press Inc., Boca Raton, Fla. pages 5 to 23, (1985)), their GRAS status and their non-pathogenicity, bacteriocins produced by these bacteria can be safely used in food systems (either produced in situ in the food system by adding the live cultures or added to food systems as cell-free purified or semi-purified or crude preparations) to extend the shelf life or to provide a margin of safety against potential food-borne pathogens. The utility of such bacteriocin-producing strains could be particularly increased if the bacteriocin-producing potential of the strains is broadened by introducing into such strains genetic information for different types of bacteriocins from closely related strains or species.
A transfer of genetic information can be achieved by conjugation, transformation (including electrotransformation), and transduction. The problem is generally, that the bacteriocins are antagonistic to the recipient Lactococcus and the recipients are either killed or the transfer does not take place.